The present invention relates to the preparation of transparent photochromic glasses displaying rapid darkening and clearing which are suitable for the production of ophthalmic lenses which have a refractive index (n.sub.d) of at least 1.585 and less than or equal to 1.610.
These glasses are also characterized by an Abbe number (.nu.) of between- about 41 and 45, a density (.rho.) of less than about 2 80 g/cm.sup.3, a good chemical stability and a viscosity at the liquidus which is sufficient to enable them to be manufactured continuously and shaped using the conventional shaping techniques (viscosity at the liquidus of greater than or equal to about 200 poises).
These glasses can also undergo the customary cycles for the deposition in vacuo of thin layers (for example anti-reflection) and can do so without a significant change in their photochromic performances.
In the present context the glasses displaying rapid darkening and clearing or fading are understood to be glasses which, at ambient temperature, under the action of a source of actinic radiation, show a light transmittance on darkening at the end of 15 minutes of less than or equal to 48%, and a light transmittance of greater than about 60% 5 minutes after having been removed from actinic radiation illumination.
The majority of patents relating to photochromic glasses described compositions enabling products, which may or may not be for ophthalmic use, to be obtained which have a refractive index of 1.523.
The advantages of ophthalmic lenses having a high refractive index are numerous. In fact, the use of glasses having a high refractive index compared with conventional glasses (n.sub.d =1.523) enables the thickness of the edge (negative power) or of the center (positive power) to be reduced, for equal power.
However, in parallel with the increase in the refractive index a reduction in the Abbe number (that is to say, an increase in the dispersion of the glass) is generally observed. In order to avoid the defects generated by the increase in the dispersion of the glass, in particular the appearance of colored fringes at the edge of the lens, the high-index glass must have an Abbe number of not less than 41.
Linked to the variations in thickness, another advantage of the high-index lenses, which is not negligible, can be a lower weight. To achieve this it is important that the density of the glass is not too high. A density of less than or equal to 2.80 g/cm.sup.3, and preferably less than 2.75 g/cm.sup.3, is generally desirable in this regard.
Moreover, the photochromic glasses having a refractive index of the order of 1.6 are products intended to be sold in most cases with surface treatments which minimize the reflective losses.
These treatments are generally carried out by known techniques for deposition under vacuum, which necessitate bringing the glass to a temperature of about 280.degree. C. It thus becomes essential that the photochromic properties of the proposed glass are not altered during this operation.
The photochromic glasses described in U.S. Pat. No. 4,891,336 (Prassas) have a high sensitivity to heat treatments carried out at temperatures between 200.degree. and 300.degree. C. The majority show a difference in light transmittance in the darkened state after exposure for 15 minutes to actinic radiation (TD.sub.15) before and after deposition of antireflective layers which can be greater than 10 points, over the range of temperatures from 0.degree. to 40.degree. C.
By way of example we give the photochromic properties of glasses without and with an anti-reflecting layer in Table I. The glass compositions are those described in Examples 21 and 24 (Table IE) of the above-mentioned patent.
TABLE I ______________________________________ Examples from U.S. Pat. No. 4,891,336 Ex. 21 Ex. 24 ______________________________________ Ag 0.147 0.147 0.146 0.146 Cl 0.232 0.232 0.230 0.230 Br 0.159 0.159 0.154 0.154 CuO 0.020 0.020 0.0230 0.0230 Sb.sub.2 O.sub.3 0.40 0.40 0.54 0.54 DEPOSIT NO YES NO YES TD.sub.20 42.4 55.7 43.0 58.9 x.sub.20 0.3437 0.3397 0.3415 0.3425 y.sub.20 0.3361 0.3361 0.3331 0.3383 T.sub.o 86.7 93.6 86.1 92.8 TD.sub.15 (0.degree. C.) 10.5 16.5 10.6 19.2 TD.sub.15 (25.degree. C.) 28.7 44.6 29.0 48.8 TD.sub.15 (40.degree. C.) 45.8 60.8 47.9 63.4 TF.sub.5 (25.degree. C.) 63.9 76.2 65.3 77.4 DT.sub.15 (40-25) 17.1 16.2 18.9 14.7 ______________________________________
As can be seen, the transmittances after darkening and clearing are 15 to 20 points higher than those of the original glass. This difference is substantially greater than that resulting from the presence of the anti-reflecting layer on its own, which is estimated to be about 4 points. A glass of this type is not acceptable.
One of the objectives of this invention is also to provide compositions in which the photochromic properties mentioned above are combined with small changes in said properties after deposition of anti-reflection layers.
More precisely, the present invention has for its primary objective to produce a photochromic lens having a high refractive index, displaying rapid darkening and clearing and capable of receiving an anti-reflection layer, said lens being formed from a glass for which the refractive index is between 1.585 and 1.610, the Abbe number is higher than 41 and the density is less than about 1.80 g/cm.sup.3, characterized in that the said lens, for a thickness of 2 mm, has the following optical properties:
(a) the light transmittance in the clear state (T.sub.o) of greater than or equal to about 85%;
(b) a light transmittance in the darkened state after exposure for 15 minutes to actinic radiation (TD.sub.15) over the temperature range from 0.degree. to 25.degree. C. of less than about 48%;
(c) a light transmittance in the darkened state after exposure for 15 minutes to actinic radiation (TD.sub.15) at 25.degree. C. of more than about 20%;
(d) the fading rate at ambient temperature (20.degree. to 25.degree. C.) such that the glass has a light transmittance (TF.sub.5) of at least 60% within 5 minutes after withdrawing from actinic radiation;
(e) the difference in light transmittance in the darkened state over the temperature range from 25.degree. to 40.degree. C. of less than 23 points of the degree of transmittance; and
(f) the difference in absolute value of light transmittance in the darkened state before and after heat treatment at 280.degree. C. for 1 hour, simulating an anti-reflection treatment, of less than 5 points of transmittance.